Water repellants (“sealants”) are commonly applied to surfaces of manufactured structural wood products in order to provide visual differentiation and to retard water absorption in the event of exposure to precipitation. Wood products treated with sealants on at least one surface include solid-sawn lumber and composites such as oriented strandboard (OSB), laminated veneer lumber (LVL), and others. In North America the sealants are typically applied to the wooden substrates on finishing lines at the primary manufacturing plant. Most sealants are water-based in order to comply with environmental and safety regulations and are made by combining a wax emulsion, a polymer latex and one or more colored pigment dispersions.
Many colored sealants for manufactured structural wood products exhibit “crocking,” which is the tendency for colored material from the sealant to be transferred to a second surface that is being rubbed against the previously applied sealant. Unfortunately, manufactured structural wood products are routinely handled and contacted during the process of packaging, transporting and use of the products during construction. Sealants that exhibit significant crocking present problems, which include unwanted transfer of colored material from the sealant to hands, gloves, boots, shirts, pants, equipment, and other objects that might contact the applied sealant on the surface of the wood product. In many cases the transferred colored material is unsightly and can be difficult to remove.
The tendency for a sealant to exhibit crocking can be easily evaluated by rubbing a white paper towel across a layer of dried sealant. If colored material transfers from the sealant to the paper towel, then the sealant is considered to exhibit crocking. Crocking is somewhat analogous to marking a paper with a crayon, which is generally comprised of colored pigment and wax. Some sealants have greater crocking potential than others. Sealants with greater crocking potential would be associated with greater color transfer in a standard rubbing procedure. Sealants that exhibit some level of crocking include CBS 197H (green), which is manufactured by the Willamette Valley Company (Eugene, Oreg., USA), and WE1460, which is manufactured by Walker Industries (Burlington, ON, CAN). In fact, essentially all colored sealants (comprised of wax emulsion, polymer latex and colored pigments) exhibit some level of crocking.
Upon application of an aqueous sealant to a wood product substrate, the water in the formulation either evaporates or absorbs into the substrate. The residual portion of the formula generally exists as a film of pigment particles dispersed in a soft, elastic matrix of wax (major component) and polymer (minor component). A relatively high wax level is required in order to impart water-repellency to the sealant. Unfortunately, the wax component is relatively soft and is particularly prone to transfer to another object via a rubbing action. The polymer component is relatively expensive and is usually less hydrophobic than the wax, but it imparts elasticity and toughness to the film and also improves wet adhesion to the wood. Higher levels of elastic polymer tend to reduce the crocking potential of the sealant, but this modification results in increased cost and reduced water repellency.
Thus, there is a need to improve the crocking potential of aqueous, colored, sealants for use in wood products without significantly increasing costs or reducing water-repellency.